There have been several incidents where a helicopter pilot forgot to lower the landing gear during a rooftop landing or offshore platform landing. Because a helideck is raised (often a few hundred feet above the ground or ocean), the existing landing gear alert function doesn't provide a timely alert. In the current ground proximity warning system (GPWS), the landing gear alert envelope is determined only by the radio altitude. The aircraft's radio altitude continues to read a large value (300 feet, for example) until the helicopter gets over the helideck. At that point, the radio altitude jumps to 20 feet, for example. Often, the helicopter is sinking toward the helideck at that point, and there may not be sufficient time for the pilot to react.